At-Grade Septic Systems

An at-grade septic system consists of a septic tank and soil absorption bed. The soil absorption bed is used to distribute the effluent (partially clarified water from septic tank) into the soil. Each at-grate contains 1.5″ washed stone, a distribution network, synthetic geo-textile fabric, and a top-soil cap. An at-grade is called an at-grade because the treatment of the effluent begins in the first inch of soil or “at grade”. A soil test reveals if an at-grade is suitable for your lot. If the limiting factor is less than 48 inches but greater than 36, than an at-grade is suitable. The absorption bed consists of stone, pipe, and synthetic fabric. First, 6 inches of stone is placed on the plowed at-grade site. The distribution network is laid on top of the stone. The distribution network is created by drilling equally spaced holes in pipe. Another 2 inches of stone is placed on top of the distribution network. Synthetic fabric is placed over the stone to protect the stone from dirt and roots. Finally, a top soil cap is placed over the entire mound area. The at-grade must maintain a 3:1 slope to prevent erosion and to divert water away from the mound. The entire at-grade is planted with grass seed.


At-grade Disbrubtion Network partially covered 300w" sizes="(max-width: 322px) 100vw, 322px" />After the initial treatment in the septic tank, the effluent travels to the at-grade component. A pump is used to control the amount of doses delivered to the at-grade and to pressurize the distribution network. After every dose, the at-grade has a “rest period” which results in superior pathogen and nutrient removal. Also, a pressurized distribution network reduces the chances for localized clogging.

The size of an at-grade is dependent on the gallons produced per day by a residence or business and the loading rate of the soil beneath the absorption bed. The gallons/day produced by a residence is based on the number of bedrooms in the house. Each bedroom is 150 gallons/day. The higher the loading rate of the soil beneath the absorption bed, the smaller the at-grade. The lower the loading rate of the soil, the larger the at-grade.

The final treatment of the effluent occurs in the soil. The soil beneath the at-grade component removes pathogens, organic matter, reduction of contaminants by aerobic microorganisms and ion bonding to negatively charged clay particles. The soil serves as a fixed porous medium on which beneficial aerobic microorganisms grow. These organisms feed on organic matter present in the waste water and help eliminate pathogens.

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